Orks have a hard time against skimmers. The best regular Orks can usually do is to take large squads so you can take some casualties, and give them 3-4 heavy weapons each. You can take Rokkits or Big Shootas, but Rokkits are normally the way to go because they at least give you a chance against the bigger AV12 skimmers, and they kill Marines too. Keep one squad behind the others so that others can Mob up with them once they finally get chewed down small enough to break. Hopefully the last few survivors will be the guys with the heavy weapons, so they stay on the table in that back squad, which now has more guns!<?
Ork shooting is more accurate than folks usually think. Three Rokkits at BS2 is an average of one hit per turn, with the chance to get lucky and get more. A SM tac squad is BS4, but only gets one ML, which means they're hitting on average 2 out of every 3 turns, and can never get more than one in a single turn. Your range is only 24", but you're always moving forward, so it doesn't take that long to get into range. Note that unlike other armies you're allowed to give your squad leaders (Nobz) an additional heavy weapon, and can also buy them ammo runts for re-rolls.
Target the Vypers first, because at AV10 open topped you have the best chance of killing/damaging them (50% chance of a kill per glancing hit). The faster you start knocking some guns out of the sky, the slower your opponent is able to chew through your boyz. Target the Wave Serpents second because they're the next best odds. They die to 1/3 of glances, and lose the main gun on another 1/6 of results. Save Falcons for last. They have the most firepower (with Starcannon on the side and Shuriken Cannon underneath 8 shots per turn, with Scatter Laser on the side sometimes more), but your opponent will ALWAYS give them a Holofield, which drastically reduces the odds of killing them to 1 out of every 9 glancing hits. You just have to hope your opponent hasn't taken too many of these and suck up the pain while you slog it out and shoot the other stuff.
If your opponent takes Crystal Targeting Matrices so he can move/shoot/move he has to pay 30pts per vehicle, which will reduce the sheer number of vehicles/guns he has. On some terrain-heavy tables, however, it will still make it virtually impossible for Orks to beat them.
Your best tool for hunting down hiding skimmers is Guntrukks armed with twin-linked Rokkits. These average a 54% chance to hit, you can take them in squads of three, and they have the speed to tear across the board from cover to cover until they get there. Being able to move 12" and fire gives you a chance to zip around a corner and get a shot.
Your best odds for dealing with the Seer Council are Slugga Boyz, Mega Armored Nobs, or possibly Big Shootas. You need to drown them in shots/attacks. Sure they save 75% of the time, but they're only T3. Compare to fighting Marines- against a Space Marine you need an average of two wounds to get a kill (thanks to Choppas), an average of three S3 hits to get a wound, and two WS4 attacks to hit. So for every 12 Slugga Boy attacks you average one dead marine. Against the Seer Council they save more often (1/4 kills per wound) but you wound more easily-1 / 2. It?s only a little worse- 16 attacks per kill (which would be the same as SM if not for Choppas), though it slows down after the Warlocks are no longer the majority models so he can use the Farseers? T4. If you can afford a squad with Big Shootas they can do decent numbers on the Seer Council. S5 against majority T3 wounds on a 2+. 12 shots averages .84 dead per turn, though you could get lucky and do better, and if you?re inside 12? you should be able to pick up another kill from your Sluggas. And as someone else suggested, you could also try a retinue of Mega-Armored Nobz. Singing Spears and Witchblades always wound on a 2+, but they allow normal armor saves, and both Farseers and Warlocks only have 1 attack base. Nobz with Mega Armor and Power Klaws have more attacks, more wounds, better saves (84% to his 75%), and should win out. The only problem is that your Trukk is very fragile, and if he manages to blow it up you?re stuck walking around as if in difficult terrain while he shoots you with Starcannons, which ignore your 2+ save. So you need to be very careful with these guys, keeping their Trukk completely hidden behind terrain until you have the opportunity to rush out, hop out, and charge all at once.
Those are probably your best bets. Remember also that the mission will make a big difference. If it requires taking objectives/quadrants, be sure to grab them and get into whatever cover you can. If it?s a straight VP battle he can whittle away at you while keeping his distance. If he has to take objectives you can force him to get close enough for you to hurt. Especially if you go second and so get the last turn of shooting/assaulting.
They should win a war of attrition against that Seer Council, no..?